(1) Field of the Invention
The present invention relates generally to an apparatus for repairing tubes of heat exchangers in a nuclear system and, more particularly, to an apparatus for expanding an existing repair sleeve in a steam generator to permit a new repair sleeve to be installed.
(2) Description of the Prior Art
The tubes of heat exchangers such as steam generators, oil coolers, reheaters or feedwater heat exchangers, in particular those of power plants, are subjected to high mechanical and thermal stresses as well as the corrosive action of the fluid which passes therethrough. The severity of the environment causes degradation of the tubes. The process of replacing tubes, particularly in power plants where down time must be limited, is both expensive and time consuming. Therefore, it is highly desirable to forestall replacement by repairing localized defects.
It is known to remove from service defective tubes by plugging them. However, plugging is only a limited remedy as it reduces the overall heat transfer surface area of the heat exchanger, and thereby materially reduces its efficiency.
Further, it is known to repair a localized tube defect by installing an inner sleeve which spans the defect. The sleeve is inserted through the end of the tube and thereafter attached near its ends to form a sealing engagement with the tube. Such expansion may be accomplished by mechanical rolling, hydraulic or explosive expansion. The sleeve can be welded into place by TIG, laser or explosive means.
A sleeve also may be used to repair a defective portion of a tube not at a tube sheet by expanding the sleeve into the tube to span the defect with a set of expansions on either side of the tube defect. In the free-span portion of the tube, where it is not backed by the tube sheet material, the risk of over expanding the tube is great if the expansion process is not tightly controlled. If an overexpansion occurs, the tube may crack either during the expansion operation or during the operation of the heat exchanger due to high stresses in the expanded region.
The acceptable leak rate between the sleeve and tube is an issue within all designs of heat exchangers. However, in radioactively contaminated heat exchangers, the leakage from the contaminated side of the tube to the clean side will result in the contamination of otherwise clean system components. In heat exchangers with dissimilar fluids on each side of the tube, such as an oil cooler, where one side of the tube is filled with water and the other with oil, leakage will result in the mixing of one fluid with the other, a potentially unacceptable situation that may result in damage to other system components. Finally, in pre-heater or feedwater heat exchangers, the leakage of fluid from one side of the tube to the other will result in the loss of efficiency of the unit. Based on the type of heat exchanger to be sleeved, an acceptable leak rate for the sleeve can be determined. In cases where the flow rate through the heat exchanger is high and the risk of contamination or damage to the system components is low, a higher leak rate may be acceptable.
In the past, once a tube was sleeved it was impossible to make another repair at the same location if the sleeve leaked. Worse still, if another leak occurred above the repair, there was no way to get a sleeve to the other leak. As a result the tube would have to be plugged. After too many tubes are plugged, the plant efficiency becomes so poor that the steam generators either must be replaced or the entire plant shutdown.
Other methods for addressing this problem have been proposed. These include the autogenous or filler weld repair of sleeves. These techniques can be very difficult to perform on degraded sleeves. Moreover, experience has shown that it is difficult to clean the area to be welded on degraded sleeves. Improper cleaning leads to problems with weld inclusions. Also, placing a sleeve within a sleeve has been proposed but the constriction that results would significantly degrade the performance of the tube.
Thus, there exists the need for an apparatus for expanding an existing repair sleeve to permit a new repair sleeve to be installed inside while, at the same time, reducing the possibility that the tube would be ruptured during the repair process.